add example HTTP(S) proxy setup to docs

[EdVinyard]

Document the process of setting up an HTTP(S) proxy in order to introduce artificial latency, bandwidth, or other network constraints to a client's conversation with Prism. For the example, consider Toxiproxy or something similarly configurable.

Background

Team Tacocat discussed adding latency/bandwidth limitations to Prism as suggested by #1140. We suggested an alternative, adding documentation that describes how to set up existing HTTP proxies that accomplish the same thing, suggested this approach, and it was well-received.

[EdVinyard]

simple example implementation at https://github.com/stoplightio/ExampleChooserPrismProxy